/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
import java.time.*;
import java.util.stream.Collectors;
 
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	class ComplexObject
	{
		ZonedDateTime time;
		ZonedDateTime getDateTime() { return time; }
	}
 
	public static void main (String[] args) throws java.lang.Exception
	{
		List<ComplexObject> objects = new ArrayList<>();
 
		Map<ZonedDateTime, List<ComplexObject>> map = objects.stream()
			.collect(Collectors.groupingBy(ComplexObject::getDateTime));
	}
}

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